1. Field of the Invention
The invention relates to an electronic controller and to the use of the electronic controller in a motor vehicle brake system.
2. Description of the Related Art
In high-quality electronic ABS and ESP brake control systems, at least some of the valve coils are no longer switched but rather analogized actuation is effected using pulse width modulated current control (PWM), which permits almost analog actuation of the hydraulic valves. For this, a plurality of valve actuation circuits are provided which, by way of example, may be designed using MOS transistors connected in phase opposition. To allow an inexpensive and space-saving solution, such a valve actuation circuit is usually implemented as an integrated circuit, especially since a complex ESP system requires up to eight such valve actuation circuits to be present in addition to numerous additional circuit parts. A pure analog amplifier for actuating a valve coil is not feasible for reasons of excessive power loss.
The procedure when measuring the actual current using a single A/D converter within a PWM controller, for actuating the aforementioned valve coils is already revealed by WO02/058967 A2 (P 10057) and WO03/039904 A2 (P 10253). On the basis of the circuit examples described therein, a particular number of current measuring channels is allocated to the A/D converter in line with the time slice principle on the basis of complex priority logic, so that the conversion capacity of said A/D converter can be used in as optimum a fashion as possible. This priority logic is relatively complex and therefore expensive.
The demands on the above electronic control units are increasing to an ever greater extent, since additional functions are also undertaken by the brake control unit or the brake systems need to have improved control quality. A few more recent control functions, including motor vehicle longitudinal control (ACC), which keeps the distance from a vehicle in front constant, require not only the mere capability of setting an analog current but also particularly precise current control, since slightest deviations from the desired current value produce palpable differences in the set braking pressure, which means that precise ACC control with an appropriate level of comfort is no longer possible. In the case of prolonged ACC control, it is also possible for just slight differences between the set pressure in the front and rear axles to result in failure of the brake function on one axle. In particular, relatively small currents in the range from approximately 100 to 400 mA need to have a high level of precision, since these currents are needed for setting small pressure differences, as are typical for longitudinal control.
In the case of valve actuation circuits which are designed in line with the aforementioned patent applications WO02/058967 A2 (P 10057) and WO03/039904 A2 (P 10253), it is therefore necessary to improve the accuracy of the PWM current control still further. In the case of PWM control based on the cited prior art, a general consideration in the usual application of brake control is that an inductive load (e.g. valve coil) is actuated. The inductive load has a determined inductance L and a nonreactive resistance R. The inductance L can be used to define a time constant for the load L/R. On the basis of this time constant and the actuating frequency of the pulse width modulation, a typical profile is obtained for the load current IL for the inductive load over time t, as shown in FIG. 1. The use of an A/D converter, which is used on a number of occasions for measuring current in different valve actuation circuits, does not allow the current to be determined at a plurality of points in the current profile in FIG. 1. The current is therefore measured at particular times (discrete-time measurements), as described in the documents cited above. Depending on the measurement time, the current value determined in this manner deviates considerably from the current's average which actually needs to be determined for the PWM control. This deviation from the average is subsequently also referred to as a form error. If, as FIG. 2 shows, the current value is regularly measured in the middle of the switched-on phase at the time tON/2, for example, then the form error shown in FIG. 2 arises as the difference between the measured value and the average.
However, the form error is influenced not only by the measurement time for the discrete current measurement but also by other operating parameters for the PWM control, such as the voltage across the load and the temperature-dependent nonreactive resistance of the load at present. Integrated analog circuits, in particular, achieve a high level of absolute accuracy only with a very high level of outlay. Although inherently known differential circuit techniques and inherently known calibration techniques, for example, allow a certain degree of independence from technological fluctuations and temperature effects, these methods are subject to limits on account of the high level of outlay. Calibrating the circuit over temperature would require very long periods of time following production and therefore holds little advantage for production in large quantities.
The current measuring principle shown in FIG. 2 requires a minimum value for the PWM signal's switched-on time so that in each period a current value can be recorded under all constraints. The result of this minimum value is that on the basis of the nonreactive resistance of the coil, the high-side voltage on the inductance and the set PWM frequency a minimum current is obtained below which no further control is possible. In the typical application of ACC control for motor vehicles, it is thus possible to regulate only currents to a minimum of 200 mA for example. However, ACC-optimized current/braking pressure characteristics for a valve coil usually require smaller currents down to approximately 100 mA.
An object of the present invention is thus to propose an electronic controller and a method for measuring current within an electronic controller, with PWM current control, which allow more accurate and safer current setting to be performed, the electronic controller needing to be relatively inexpensive, in particular.